Several radiography diagnostic and screening techniques depend on the visualization of small objects having a high attenuation coefficient embedded in a cluttered soft-tissue environment. These include, but are not limited, coronary angiography (where a contrast agent is added to blood vessels in the heart to assess cardiovascular diseases) and calcification detection in chest radiography (where the benignity of pulmonary nodules is characterized by its calcium content). Enhancing the detectability of these objects is, therefore, very desirable.
A technique that has long been proposed to achieve this is that of dual-energy (DE) subtraction imaging, which exploits the different energy-dependence of the X-ray attenuation coefficient of different tissue types to remove the soft-tissue component from the radiographic image and hence enhance visualization of the objects of interest.
DE systems work by obtaining one low-energy and one high-energy image, and performing a weighted subtraction to combine them into a final image (for display) in which the cluttered soft-tissue structure has been removed.
An alternative DE imaging method is the single-shot approach, in which both images are obtained simultaneously. This is accomplished by stacking two sensor layers vertically to form a two-layer detector in what is known as a sandwich configuration. The spectral separation results from a combination of the intrinsic higher probability of high-energy photons to go through the top layer unabsorbed and the presence of a metal beam-hardening mid-filter in between the two layers. Single-shot techniques are thus much more tolerant to both patient and cardiac motion than kVp switching, and are compatible with most current X-ray sources. However, the presence of the hardening mid-filter means that a portion of the X-rays are wasted in it, resulting in patient dose inefficiency.
Therefore, there is provided a novel X-ray imager that overcomes disadvantages of current systems.